Thermal radiation shielding



Nov. 5, 1968 B. T. EBIHARA 3,409,730

THERMAL RADIATION SHIELD ING Filed June 17, 1966 INVENTOR BEN T. EBIHARA BY q Q ATTORNEYS United States Patent 3,409,730 THERMAL RADIATIONSHIELDING Ben T. Ebihara, Strongsville, Ohio, assignor to the UnitedStates of America as represented by the Administrator of NationalAeronautics and Space Administration Filed June 17, 1966, Ser. No.559,349 7 Claims. (Cl. 13-35) ABSTRACT OF THE DISCLOSURE Spacedradiation barriers of refractory metal foil encircle an inductionfurnace. A mat of refractory metal fibers maintain the spacing betweenthese radiation barriers.

Statement of government ownership The invention described herein wasmade by an employee of the United States Government and may bemanufactured and used by or for the Government for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention is concerned with increasing the operating range of aninduction furnace. More particularly, the invention relates to improvedthermal radiation shielding for an induction furnace or other hightemperature device.

The operating range of an induction furnace having a limited powersupply and cooling capacity can be improved by more efiicient shieldingof the coil, furnace components, and surrounding atmosphere from theradiant heat that escapes from the metal being inductively heated.Ceramics have been customarily used as insulating materials in inductionfurnaces, but a ceramic material is limited to low service temperaturesbecause of its melting point, resistivity, or chemical incompatibility.

For higher temperature applications multiple shielding with a compositeof refractory metal sheets and ceramic oxide has been used to about 4500F. However, most ceramics are sensitive to thermal shock which preventsrapid cycling of the temperature. Also, certain oxides having highertemperature capabilities above 3000 F. are toxic and radioactive.Because of these shielding and insulating problems, the operation ofsuch induction furnaces at temperatures around 5500 F. has beenextremely limited.

Tungsten has been proposed as a shielding material, but the greater carerequired in the handling and fabrication of this material complicatesboth the design and manufacture of the insulation. Tungsten is hard andbrittle at room temperatures, and its fabrication requires experiencedpersonnel as well as special machines and tools. When heated above 2000F. by welding or otherwise tungsten becomes extremely brittle fromrecrystallization.

These problems have been solved by thermal radiation shieldingconstructed in accordance with the present invention. This shieldingutilizes refractory metal foil laminated or alternated with cleanrefractory metal fine fibers. This insulation makes use of the principleof multiple radiation barriers to reduce the transfer of heat. Thefibers serve as a low-conductivity spacer between these barriers.

It is, therefore, an object of the present invention to provide animproved thermal radiation shielding which utilizes all metalconstruction in the form of both foil and fibers.

Another object of the invention is to provide improved thermal radiationshielding for insulating an induction furnace which exhibits a minimumof susception in an induction field and has low thermal inertia togetherwith insensitivity to thermal shock.

A further object of the invention is to provide a light and compactshielding which can be fabricated by inexperienced personnel withoutspecial tools or equipment.

These and other objects of the invention will be ap parent from thespecification which follows and from the drawing.

The drawing is a perspective view of refractory metal foil and fiber matcomposite shielding in a cylindrical configuration.

Referring now to the drawing, there is shown a composite form ofimproved thermal radiation shielding 10 constructed in accordance withthe invention. The composite shielding structure 10 is spirally wrappedin the form of a cylinder to the desired thickness. The cylindricalshield 10 is adapted to encircl an induction furnace to contain theradiation from the furnace until it approaches the melting temperatureof the shielding material. If desired, the diameter of the shielding 10can be made small enough so that it will fit inside the furnace coil.

The shielding 10 includes a radiation shield 12 in the form of metalfoil. The adjacent wraps of the foil 12 are separated by a spacer 14.

Tungsten foil is used for the radiation shield 12. One to two milstungsten foil has been satisfactory for the shield 12, and the thinnessof the foil prevents or strongly inhibits electrical susception in theinduction field of the furnace. While tungsten is the preferred materialfor the foil shield 12, other materials, such as rhenium, tantalum,molybdenum, or platinum, can be used so long as the material is madeinto thin flexible foil sheets.

This shield 12 has a relatively short outgassing time when used invacuum furnaces. This outgassing is primarily caused by absorbed gas onthe surfaces rather than that absorbed deeply within the material.

The spacer 14 is in the form of a fine metal fibrous mat, and foroptimum results at extremely high temperatures the metal fibers in themat must be of the same metal as the foil in the shield 12 to achievechemical compatibility. Tungsten fibers having a diameter of one to twomils have been found to be highly satisfactory for this purpose. Thesefibers are especially useful as spacers because of their lowconductivity between the layers of foil caused by the loose packing ofthe fibers.

After the foil 12 and spacer mat 14 have been rolled into the insulatingcomposite 10 a plurality of binding wires 16 are utilized to retain thecylindrical shap for subsequent mounting adjacent an induction furnace.This construction achieves an assembly of multiple radiation barriersand a minimum of space. This compactness is important in increasing theefficiency of the furnace induction coils while making possible closercoupling. Also the foil and fibrous mat are extremely light in weight.

Thermal radiation shielding constructed in accordance with the inventionwas tested in a vacuum induction furnace having a cylindrical interior3.5 inches in diameter and 8 inches long to ascertain its value. Atungsten specimen having a diameter of two inches and a length of sixinches was enclosed by a cylindrical shield 10. The furnace produced afinal temperature of 5850 F. on the specimen.

Because of its simplicity, the insulation composite 10 can be fabricatedvery easily by inexperienced personnel without special tools orequipment. The bulk density of the shielding reduces the thermal inertiafor faster heat up and cool down time of the furnace. Also, thetemperature can be cycled rapidly without high thermal stress.

While a preferred embodiment of the invention has been shown anddescribed, it will be appreciated that various modifications can be madeto the thermal radiation shielding without departing from the spirit ofthe invention or the scope of the subjoined claims. For example,while'the lamination of foil 12 and fibers 14 is shown in the drawing asbeing wrapped spirally in a cylindrical shape to the desired thickness,it will be appreciated that the various layers can be wrappedconcentrically prior to banding.

What is claimed is:

1. Thermal radiation shielding for an induction furnace to heat aspecimen to an extremely high temperature comprising:

spaced refractory metal radiation barriers encircling the inductionfurnace adjacent thereto, and

spacers having a low thermal conductivity interposed between saidradiation barriers to maintain the spacing therebetween, said spacersbeing fibers of the same refractory metal as said radiation barriers.

2. Thermal radiation shielding as claimed in claim 1 wherein theradiation barriers are foil.

3. Thermal radiation shielding as claimed in claim 2 wherein therefractory metal foil is extremely thin to inhibit electrical susceptionin the induction field of the furnace.

4. Thermal radiation shielding as claimed in claim 3 wherein the foilhas a thickness of about one to two mils.

5. Thermal radiation shielding as claimed in claim 1 wherein the fibershave a diameter of about one to two mils.

6. Thermal radiation shielding as claimed in claim 5 wherein the fibersare in the form of mat.

7. Thermal radiation shielding as claimed in claim 6 wherein the fibrousmat composes tungsten fibers to enable the specimen to be heated toabout 5500 F.

References Cited UNITED STATES PATENTS 2,308,945 1/1943 Van Embden 13-262,476,916 6/1949 Rose et al. 263-50 3,294,513 12/1966 Beattie 263-503,327,041 6/1967 Clune et al. 26350 3,355,537 11/1967 Troellet a1. 13-35RICHARD M. WOOD, Primary Examiner. L. H. BENDER, Assistant Examiner.

